This application claims priority of Korea patent Application No. 99-63712, filed on Dec. 28, 1999.
(a) Field of the Invention
The present invention relates to a gasoline direct injection (GDI) engine, and more particularly, to an injection pressure control method of a GDI engine.
(b) Description of the Related Art
Generally, internal combustion engines are operated by supplying an air/fuel mixture into a cylinder, and compressing and igniting the mixture. A procedure for generating power in internal combustion engines to drive a vehicle comprises the steps of supplying air through an air supply system, injecting fuel such that it can mix with the air during an intake stroke, injecting the air-fuel mixture into a vaporization portion, igniting the mixture using a spark plug, and exhausting burned gas through an exhaust system.
Recently, much research and development has been pursued for improving fuel consumption and reducing emissions utilizing direct injection internal combustion engines.
Generally, a direct injection engine adopts a swirl air intake mechanism for quickly producing an air/fuel mixture and directly injects fuel into the combustion chamber at a predetermined pressure according to the valve timing of the valve system.
FIG. 3 schematically shows a conventional direct fuel injection mechanism. The fuel stored in a fuel tank 112 having a pressure of about 3 bar is increased in pressure to about 120 bar by a high-pressure pump 114, then supplied to a fuel rail 118 through a fuel pipe 116, and injected into a combustion chamber 122 through an injector 120.
At this point, pressure of the fuel supplied to the fuel rail 118 is detected by a pressure sensor 124, then output to an electronic control unit (ECU) which is not shown in the drawing. The ECU controls a pressure control valve 126 according to the detected pressure such that the pressure control valve 126 adjusts the fuel pressure in a range of 40-120 bar according to engine operation modes (i.e., a stratified charge mode and a homogenous charge mode). For example, the ECU controls the pressure control valve 126 such that the fuel pressure is reduced to prevent fuel concentration in the stratified charge mode and is increased to prevent dispersion of the fuel in the homogenous charge mode.
Generally, in such direct fuel injection engines, a continuous variable valve timing (CVVT) mechanism is employed to continuously vary the valve timing and the valve lift according to the drive conditions of the vehicle. Accordingly, the effective compression ratio within a combustion chamber is varied together with the variation of intake valve timing as the injection is realized during a compression stroke in the stratified mode.
In this case, as only the engine torque and the engine RPM are considered to determine the injection pressure in the stratified charge mode, the fuel injection cannot be optimally performed. That is, since the variation of the effective compression ratio in a combustion chamber is not considered, an appropriate fuel injection amount and time cannot be realized, resulting in increased emission gasses and degraded driving quality.
Therefore, the present invention has been made in an effort to solve the above problems.
It is an objective of the present invention to provide an injection pressure control method for a GDI engine capable of optimizing a fuel injection amount by setting a pressure value in consideration of a compression ratio variation rate in a combustion chamber when a variable valve timing mechanism is employed.
To achieve the above objective, the present invention provides an injection pressure control method of a GDI engine comprising the steps of detecting an engine torque and an engine rpm; setting an initial stratified mode fuel pressure value according to the detected engine torque and engine rpm; calculating a weighting factor according to a variation rate in effective pressure of a combustion chamber of the engine; establishing a corrected stratified mode fuel pressure value by multiplying the wighting factor to the initial stratified mode fuel pressure value; comparing the corrected stratified mode fuel pressure value with a current fuel pressure value; and calculating a final fuel pressure value according to the comparision result.
The step of calculating the weighting factor comprises the steps of dividing a variable compression section of a crankshaft rotation angle into a predetermined number of unit sections; detecting a variation ratio in the effective pressure in the unit sections; and setting the variation ratio as the weighting factor.
The variable compression section is within a range of 40-60xc2x0 of a crank angle after bottom dead center.
The predetermined number of unit sections is about 20.